An example (see patent document 1) of the configuration of a conventional coordinate position detection device is schematically shown in FIG. 5. The conventional coordinate position detection device shown in FIG. 5 is a coordinate position detection device for use in a capacitive-coupling touch panel. The capacitive-coupling touch panel has a position-detection resistive film; electrodes A to D are formed in the four corners of the position-detection resistive film. The conventional coordinate position detection device shown in FIG. 5 is provided with resistors RA to RD, current-variation detection circuits 21A to 21D, analog signal processing circuits 22A to 22D, detection filtering circuits 23A to 23D, noise-elimination direct-current conversion circuits 24A to 24D, a control device 25 and an alternating-current power supply 26.
The current-variation detection circuits 21A to 21D detect variations in currents flowing between the electrodes A to D and ground when in the position detection mode. An alternating-current voltage is applied to the electrodes A to D by the alternating-current power supply 26. Thus, currents through the electrodes A to D caused by the touch of a finger or the like have an alternating-current component.
The outputs of the current-variation detection circuits 21A to 21D are amplified and band-pass filtered by the analog signal processing circuits 22A to 22D. The outputs of the analog signal processing circuits 22A to 22D are detected by the detection filtering circuits 23A to 23D, and are then fed to the noise-elimination direct-current conversion circuits 24A to 24D. The noise-elimination direct-current conversion circuits 24A to 24D convert the outputs of the detection filtering circuits 23A to 23D into direct currents to generate signals commensurate with the currents through the electrodes A to D, and feed the generated signals to the control device 25. The control device 25 detects the coordinate position of a touched area based on the outputs of the noise-elimination direct-current conversion circuits 24A to 24D.
When the conventional coordinate position detection device shown in FIG. 5 is in the position detection mode, an alternating-current voltage is constantly applied to the electrodes A to D by the alternating-current power supply 26. Thus, when the capacitive-coupling touch panel is touched by a finger or the like while in the position detection mode, an alternating current is constantly passed from the alternating-current power supply 26 via the finger or the like to ground. This disadvantageously results in increased power consumption.
The applicant of the present invention has already applied for a patent (Japanese Patent Application No. 2004-072073) on an invention that relates to a coordinate position detection device that can overcome such a disadvantage. The configuration of the coordinate position detection device proposed in this patent application is shown in FIG. 6.
The coordinate position detection device shown in FIG. 6 is a coordinate position detection device for use in a capacitive-coupling touch panel 40. The capacitive-coupling touch panel 40 has a position-detection resistive film 41. Parasitic capacitances Ca and Cb are present in the capacitive-coupling touch panel 40.
The coordinate position detection device shown in FIG. 6 is provided with compensation circuits 31 and 33, charging circuits 32 and 34, current-to-voltage conversion circuits 35 and 36, sampling circuits 37 and 38 and a control device 39 that detects the coordinate position of a touched area based on the result of the sampling fed from the sampling circuits 37 and 38. For ease of explanation, in FIG. 6, a linear resistor is shown as the position-detection resistive film 41; however, in an actual touch panel, the position-detection resistive film 41 extending two-dimensionally works similar to the linear resistor.
A resistance r1 refers to a resistance between the position TP of an area touched by a finger or the like and the left end of the position-detection resistive film 41; a resistance r2 refers to a resistance between the position TP of the area touched by a finger or the like and the right end of the position-detection resistive film 41. An impedance 42 refers to an impedance (an impedance of a user touching the touch panel 40) between the position TP of the area touched by a finger or the like and ground. The ground potential is assumed to be V0 volts.
For ease of explanation, examples of the circuit configurations of circuit blocks in one system (the compensation circuit 31, the charging circuit 32, the current-to-voltage conversion circuit 35 and the sampling circuit 37) are only shown; the other system (the compensation circuit 33, the charging circuit 34, the current-to-voltage conversion circuit 36 and the sampling circuit 38) has the same circuit configurations. Hereinafter, a description will be given of the system (the compensation circuit 31, the charging circuit 32, the current-to-voltage conversion circuit 35 and the sampling circuit 37).
The charging circuit 32 has both the capability of charging a coupling capacitance (a capacitance formed by the touch of a finger or the like between the position-detection resistive film 41 and ground) and the capability of restoring the charged coupling capacitance to its state prior to charging. The charging circuit 32 has a P-channel MOS transistor P1 and an N-channel MOS transistor N1. The P-channel MOS transistor P1 and the N-channel MOS transistor N1 are turned on and off by control signals from the control device 39.
Before the start of charging of the coupling capacitance, the P-channel MOS transistor P1 is off, and the N-channel MOS transistor N1 is on. Thus, a voltage at a terminal T1 is V0 volts.
Thereafter, the P-channel MOS transistor P1 is turned on, and the N-channel MOS transistor N1 is turned off, and thus the voltage at the terminal T1 becomes equal to that at a terminal T2 in the current-to-voltage conversion circuit 35. Here, since the voltage at the terminal T2 is set at V0+VREF volts, the voltage at the terminal T1 becomes V0+VREF volts. Thus, a charging current i1 of the coupling capacitance is fed to the current-voltage conversion circuit 35 through the P-channel MOS transistor P1.
After the completion of sampling by the sampling circuit 37, the P-channel MOS transistor P1 is turned off, and the N-channel MOS transistor N1 is turned on. Thus, the charged coupling capacitance is restored to its state prior to charging, and it remains in this state.
The current-to-voltage conversion circuit 35 has both the capability of converting the charging current of the coupling capacitance into a voltage and the capability of restoring the voltage obtained by converting the charging current of the coupling capacitance to its state prior to charging. The current-to-voltage conversion circuit 35 has P-channel MOS transistors P2 and P3, N-channel MOS transistors N2 and N3, capacitors C1 and C2, an operational amplifier OP 1 and a voltage source VS1. The P-channel MOS transistors P2 and P3 and the N-channel MOS transistor N2 and N3 are turned on and off by control signals from the control device 39. The voltage source VS1 outputs a reference voltage VREF.
Before the start of charging of the coupling capacitance by the charging circuit 32, the P-channel MOS transistors P2 and P3 and the N-channel MOS transistors N2 and N3 are on, and thus a voltage at the terminal T2 is set at V0+VREF volts, and a voltage across the capacitor C1 is set at 0 volts.
Thereafter, the P-channel MOS transistors P2 and P3 and N-channel MOS transistors N2 and N3 are turned off, and thus the voltage at the terminal T2 is kept at V0+VREF volts. When the charging of the coupling capacitance by the charging circuit 32 is started, the capacitor C1 is charged by the current i1 fed into the current-to-voltage conversion circuit 35. Thus, a voltage corresponding to charges in the charged capacitor C1 is outputted via a terminal T3.
After the completion of sampling by the sampling circuit 37, the P-channel MOS transistors P2 and P3 and the N-channel MOS transistors N2 and N3 are turned on. Thus, the voltage across the capacitor C1 is set at 0 volts, and the voltage outputted via the terminal T3 is restored to its state prior to the charging of the coupling capacity and it remains in this state.
The sampling circuit 37 samples the voltage obtained by converting the charging current of the coupling capacity, and the result of the sampling is fed to the control device 39. The sampling circuit 37 has a P-channel MOS transistor P4, an N-channel MOS transistor N4 and a capacitor C3. The P-channel MOS transistor P4 and the N-channel MOS transistor N4 are kept on while the sampling is performed, and are kept off while the sampling is not performed.
The compensation circuit 31 compensates for the effect of the parasitic capacitance Ca in the touch panel 40. The compensation circuit 31 has a P-channel MOS transistor P5, an N-channel MOS transistor N5 and a compensation capacitance Cc. A voltage of V0+VREF×2 volts is applied to the source of the P-channel MOS transistor P5.
When the P-channel MOS transistor P1 in the charging circuit 32 is off, and the N-channel MOS transistor N1 in the charging circuit 32 is on, the voltage across the parasitic capacitance Ca in the touch panel 40 is 0 volts. At this time, the P-channel MOS transistor P5 in the compensation circuit 31 is on, and the N-channel MOS transistor N5 in the compensation circuit 31 is off, and this allows the compensation capacitance Cc to be charged. After the completion of the charging, the voltage across the compensation capacitance Cc is VREF×2 volts.
When the P-channel MOS transistor P1 in the charging circuit 32 is on, and the N-channel MOS transistor N1 in the charging circuit 32 is off, the parasitic capacitance Ca in the touch panel 40 is charged. At this time, the P-channel MOS transistor P5 in the compensation circuit 31 is turned off, and the N-channel MOS transistor N5 in the compensation circuit 31 is turned on, and this allows the compensation capacitance Cc to be discharged. The voltage across the parasitic capacitance Ca after the completion of the charging and the voltage across the compensation capacitance Cc after the completion of the discharging are each VREF volts. Hence, when the parasitic capacitance Ca and the compensation capacitance Cc are set at the same capacitance, the charging current i3 of the parasitic capacitance Ca and the discharging current i3 of compensation capacitance Cc can be made equal in magnitude to each other. Thus, with the compensation capacitance Cc, it is possible to compensate for the effect of the parasitic capacitance Ca.
Since a voltage of V0+VREF volts is simultaneously applied to both the left and right ends of the position-detection resistive film 41 by the charging circuits 32 and 34, the ratio of the charging current i1 to the charging current i2 is expressed in equation (1) below.i1:i2=r2:r1  (1)
Since the ratio of a voltage V35 outputted from the current-to-voltage conversion circuit 35 to a voltage V36 outputted from the current-to-voltage conversion circuit 36 is equal to that of the charging current i1 to the charging current i2, equation (2) below holds true.V35:V36=i1:i2=r2:r1  (2)
Since equation (2) above holds true, the control device 39 can determine the ratio of the resistance r1 to the resistance r2 based on the result of the sampling fed from the sampling circuits 37 and 38, and can detect the coordinate position of a touched area from the ratio of the resistance r1 to the resistance r2.
The coordinate position detection device shown in FIG. 6 can detect the coordinate position of a touched area on a minimum of a single charge of the coupling capacitance. Thus, the relevant circuits are operated intermittently by keeping them off while the detection is not performed. In this way, it is possible to reduce power consumption as compared with the conventional coordinate position detection device shown in FIG. 5.
Patent document 1: JP-A-2003-066417 (paragraphs 0066 to 0068, FIG. 6)
In a case where the position-detection resistive film 41 in the touch panel 40 is disposed opposite an opposite conductive film (a conductive film opposite an active matrix substrate) in a liquid crystal panel, and an alternating-current voltage is applied to the opposite conductive film in the liquid crystal panel, however, the accuracy with which the coordinate position of a touched area is detected disadvantageously decreases in the coordinate position detection device shown in FIG. 6. Likewise, when the compensation circuit 31 shown in FIG. 6 is not used, the accuracy with which the coordinate position of a touched area is detected disadvantageously decreases. Such disadvantages will be described in detail below.
In a case where a pulsed alternating-current voltage is applied to the opposite conductive film in the liquid crystal panel, as shown in FIG. 7, a potential Va at an opposite electrode varies per horizontal synchronization period TH. Such variations in the potential Va at the opposite electrode produce an induced voltage in the position-detection resistive film 41 disposed parallel to the opposite electrode, with the result that a potential Vb at the position-detection resistive film 41 varies as shown in FIG. 7. When the potential Vb at the position-detection resistive film 41 varies in this way, the discharge current passed when the compensation capacitance Cc is discharged varies depending on the timing of discharge. Specifically, the amount of the current discharged from the compensation capacitance Cc when the potential at the position-detection resistive film 41 is positive is different from the amount of the current discharged from the compensation capacitance Cc when the potential at the position-detection resistive film 41 is negative. Consequently, the accuracy with which the coordinate position of a touched area is detected decreases.
A feature of an example embodiment presented herein is to provide a coordinate position detection device for use in a touch panel that can improve the accuracy with which the coordinate position of a touched area is detected even when the potential at a position-detection resistive film periodically varies.
To achieve the above feature, according to the present embodiment, a coordinate position detection device for use in a capacitive-coupling touch panel having a position-detection resistive film whose potential periodically varies between a first potential range (for example, positive) and a second potential range (for example, negative) is provided, and the coordinate position detection device includes: a current supply section passing a current through a coupling capacitance in the capacitive-coupling touch panel; a restoration section restoring the coupling capacitance having the current supplied thereto to its state prior to the supply of the current; a conversion section converting into a voltage a total amount of currents supplied by the current supply section to the coupling capacitance after the current supply section supplies the current to the coupling capacitance a plurality of times by repeating a current supply operation of the current supply section and a restoration operation of the restoration section; and a computation section detecting the position of a touched area based on the output of the conversion section. Here, the number of times that the current supply section supplies the current to the coupling capacitance when a potential at the position-detection resistive film falls within the first potential range is substantially equal to the number of times that the current supply section supplies the current to the coupling capacitance when the potential at the position-detection resistive film falls within the second potential range.
With this configuration, it is possible to reduce variations (for example, variations in a current discharged from a compensation capacitance if a compensation circuit is provided in the coordinate position detection device) attributable to the variation of the potential at the position-detection resistive film. This makes it possible to improve the accuracy with which the coordinate position of a touched area is detected even when the potential at the position-detection resistive film periodically varies. When a total of the number of times that the current supply section supplies the current to the coupling capacitance when the potential at the position-detection resistive film falls within the first potential range and the number of times that the current supply section supplies the current to the coupling capacitance when the potential at the position-detection resistive film falls within the second potential range is an odd number, the total number of times is preferably five or more. As the total number of times is increased, the ratio of the difference between the number of times that the current supply section supplies the current to the coupling capacitance when the potential at the position-detection resistive film falls within the first potential range and the number of times that the current supply section supplies the current to the coupling capacitance when the potential at the position-detection resistive film falls within the second potential range to the total number of times is increasingly reduced. Thus, it is possible to further improve the accuracy with which the coordinate position of a touched area is detected.
The number of times that the current supply section supplies the current to the coupling capacitance when the potential at the position-detection resistive film falls within the first potential range may be equal to the number of times that the current supply section supplies the current to the coupling capacitance when the potential at the position-detection resistive film falls within the second potential range.
With this configuration, it is possible to cancel out the variations attributable to the variation of the potential at the position-detection resistive film. This makes it possible to further improve the accuracy with which the coordinate position of a touched area is detected even when the potential at the position-detection resistive film periodically varies.
The current supply operation in which the current supply section supplies the current to the coupling capacitance when the potential at the position-detection resistive film falls within the first potential range and the current supply operation in which the current supply section supplies the current to the coupling capacitance when the potential at the position-detection resistive film falls within the second potential range may be performed alternately.
A period during which the potential at the position-detection resistive film falls within the first potential range may be equal to a period during which the potential at the position-detection resistive film falls within the second potential range.
When a point in time when the potential at the position-detection resistive film is shifted between the first potential range and the second potential range is a reference point, the current supply operation of the current supply section may be started with a predetermined timing from the reference point. Thus, the potential at the position-detection resistive film does not vary in each current supply operation, that is, it becomes stable. This makes it possible to further improve the accuracy with which the coordinate position of a touched area is detected. The current supply operation of the current supply section may be started immediately before the reference point, and the current supply operation of the current supply section may be completed at the reference point. Thus, variations in the potential at the position-detection resistive film are reduced during each current supply operation. This makes it possible to further improve the accuracy with which the coordinate position of a touched area is detected.
With a coordinate position detection device according to the present embodiment, it is possible to improve the accuracy with which the coordinate position of a touched area is detected even when the potential at a position-detection resistive film periodically varies.